Natural gas (NG) is stored as a liquefied natural gas (LNG) for facility in transportation and storage, or the like, and is used mainly for thermal power generation or for city gas after being vaporized. Then, a technique of effectively utilizing the cold of LNG is developed. Generally, as equipment for liquefying nitrogen gas or the like by using the cold of LNG, a process is used such that nitrogen gas is compressed by a compressor up to a pressure such that the nitrogen gas can be liquefied by heat exchange with the LNG, and subsequently the nitrogen gas is subjected to the heat exchange with the LNG in a heat exchanger to vaporize the LNG by raising the temperature and to liquefy the nitrogen gas.
Also, with respect to the electric power for driving the compressor, the tariff at night is set to be lower than the tariff for daytime, so that a gas liquefying process for efficiently liquefying a gas while taking the fluctuation of the supply amount of the above LNG and the difference in the electric power tariff into consideration is proposed. For example, referring to FIG. 7, there is known a method of liquefying a gas by using the cold of liquefied natural gas by a liquefaction process provided with at least one gas compressor 101, at least one gas expansion turbine 103, and a heat exchanger 102 for performing heat exchange between the gas and the liquefied natural gas, in which the aforesaid expansion turbine 103 is stopped or operated in a decreased amount when the supplied liquefied natural gas increases in amount, while the aforesaid expansion turbine 103 is started or operated in an increased amount when the supplied liquefied natural gas decreases in amount (See, for example, JP-A-05-45050).
However, with an apparatus for producing a low-temperature liquefied fluid or the like such as described above, various problems such as the following occurred in some cases.
(i) The amount of LNG supplied to the gas liquefying process may generally fluctuate due to the fluctuation in the demand for thermal power generation, city gas, or the like, and the amount of cold that can be used may also fluctuate. Therefore, there is a demand for an apparatus or a method by which the cold of LNG can be efficiently used so that the amount of production of the liquefied fluid or the like may not be affected even when the supplied LNG decreases in amount.
(ii) In order to pressurize a gas having a normal temperature and a normal pressure in a process for producing a compressed gas, addition of a large amount of energy and the cold for restraining the gas temperature rise accompanying the compression will be needed. In producing a compressed gas for general use that is consumed in a large amount, such as a nitrogen gas, there is a big problem for an efficient use of the cold and a comprehensive reduction of energy.
(iii) With respect to the temperature at which a gas having a normal pressure starts being liquefied, the temperature is about −80° C. for LNG, while the temperature is about −120° C. for nitrogen. For example, in a process for liquefying nitrogen gas at a normal pressure using LNG as the cold, in a state in which the liquefaction of nitrogen has started, the LNG that is subject to heat exchange with this nitrogen is still in a liquid state having a large latent heat, so that, in view of this process alone, the cold of the LNG is not sufficiently used. Also, it is not necessarily easy to use the cold of the residual LNG for other purposes, so that there is a big problem for an efficient use of energy including the cold of LNG in such a liquefaction process.